1. Field of the Invention
The present invention relates generally to medical devices, systems, and methods. More particularly, the present invention relates to the measurement of a tissue surface such as the surface of the cornea. The invention allows measurement of the tissue surface shape, and/or can provide a measurement of the hydration of the tissue.
Measurements of the surfaces of the eye are useful in diagnosing and correcting vision disorders. Refractive vision errors such as nearsightedness, farsightedness and astigmatism may be corrected surgically. Photorefractive keratectomy (PRK) and phototherapeutic keratectomy (PTK) employ optical beam delivery systems for directing a pattern of laser energy to a patient""s eye in order to selectively ablate corneal tissue to reform the shape of the cornea and improve vision. These techniques generally sculpt the corneal tissue to alter the optical characteristics of the eye. Measurement of the eye surface may enhance the accuracy of the sculpting procedure, and could be used to verify that resculpting is proceeding as intended.
Known laser eye surgery techniques often rely on an analysis of the patient""s vision to calculate a predetermined pattern of the laser energy so as to effect a desired change in the optical characteristics of the eye. These calculations often assume that the corneal tissue ablates uniformly. The laser pattern is often defined by a beam formed as a series of discrete laser pulses, and known pulse pattern calculation algorithms often assume that each pulse of laser energy removes corneal tissue to a uniform depth, so that the size, location, and number of pulses distributed across the target region of the corneal tissue determine the characteristics of the resculpting. Such techniques work quite well, particularly for eyes having xe2x80x9cregularxe2x80x9d refractive errors such as myopia, hyperopia, astigmatism, and the like. However, work in connection with the present invention has suggested that pulse ablation depths are not always uniform. Additionally, treatment of irregular corneas can benefit significantly from an accurate measurement of the corneal surface shapes. Hence, a combination of refractive resculpting capabilities with techniques for accurately measuring the shape of the eye would appear to be quite promising.
Current techniques for measuring the eye during surgery suffer from various limitations. Generally, known techniques for measuring the shape of an eye measure either light that is reflected from the surface of the eye, light that scatters from the eye, or the fluorescence of a dye that is applied to the eye. Unfortunately, the surface of the cornea becomes rough during surgery. Light that is reflected from the eye is unevenly scattered, often making measurements with reflected light difficult and inaccurate. Many techniques that employ scatter from the surface of the eye also have limited accuracy because light does not scatter evenly from the rough eye surface. Applying a fluorescent dye to the eye can lead to an inaccurate measurement of the surface shape because it is the shape of the dye covering the eye, rather than the eye itself, that is measured. Also, applying a dye to a tissue structure of the eye can delay a surgical procedure, and generally changes the hydration of the eye.
Hydration of the eye can also be difficult to measure accurately using known techniques, particularly during an ablation procedure. As both the depth of an ablation and the shape of tissue removed can vary with the water content of the tissue, known laser eye surgery techniques often include provisions to control the moisture in the corneal tissue before and/or during the procedure. Nonetheless, variations in moisture content, both locally (on different areas of the same target tissue) and between different patients (in different climates, or the like) can occur, potentially leading to significant differences between the intended resculpting and the actual change in the shape of the corneal tissue.
In light of the above, it would generally be desirable to provide improved tissue surface measurement and ablation systems, devices, and methods. It would be beneficial if the improved surface measurement techniques were suitable for integration with known laser eye surgery systems, particularly if these techniques could provide diagnostic information before, and/or feedback information during, a corneal resculpting procedure. It would further be beneficial to provide information on the shape and/or hydration of the corneal surface itself, and if these measurements could be used to modify the resculpting laser energy pattern for that corneal tissue surface. Some or all of these objectives are satisfied by the devices described below.
2. Description of the Background Art
Techniques for measuring the surface of the cornea using a film covering the cornea are described in U.S. Pat. Nos. 3,169,459; 4,761,071; 4,995,716; and 5,159,361. Moire techniques using specular reflection from the surface of the eye or fluorescent dyes are described in U.S. Pat. Nos. 4,692,003; 4,459,027; and 5,406,342. A technique for measuring the surfaces of the cornea using a vidicon tube is described in U.S. Pat. No. 4,019,813.
A technique for measuring the eye during laser eye surgery is described in U.S. patent application Ser. No. 09/083,773, entitled xe2x80x9cSystems and Methods for Imaging Corneal Profilesxe2x80x9d, filed on May 22, 1998. Techniques for combining corneal topography and laser eye surgery are described in U.S. Pat. No. 4,669,466 and 4,721,379, respectively entitled xe2x80x9cMethod And Apparatus For Analysis And Correction Of Abnormal Refractive Errors Of The Eyexe2x80x9d and xe2x80x9cApparatus For Analysis And Correction Of Abnormal Refractive Errors Of The Eye.xe2x80x9d An exemplary system and method for treating irregular corneas is described in U.S. patent application Ser. No. 09/287,322, entitled xe2x80x9cOffset Ablation Profiles For Treatment Of Irregular Astigmatismxe2x80x9d, filed on Apr. 7, 1999 now U.S. Pat. No. 6,245,059.
Each of the above references is herein incorporated by reference in its entirety.
The present invention generally provides improved systems, devices, and methods for measuring and/or changing the shape of a tissue surface, particularly during laser eye surgery. The invention generally takes advantage of fluorescence of the tissue at and immediately underlying the tissue surface. Preferably, the excitation energy will be in a form which is readily absorbed by the tissue within a small tissue depth from the surface to be measured, thereby enhancing the resolution of any surface topography measurements. Conveniently, the excitation light energy to induce this fluorescence may be provided by the same source used for photodecomposition of the tissue. Hence, these measurement techniques may be readily incorporated into laser eye surgery systems and procedures, providing surface shape information before, during, and/or after a resculpting of the cornea. The invention may optionally take advantage of changes in the fluorescence spectrum of a tissue which occur in correlation with changes in the tissue""s hydration. Such hydration measurements may be used to revise the ablation algorithm locally and/or globally throughout the treatment region, enhancing the accuracy of the ablation energy pattern by compensating for the changes in ablation rates due to variation in hydration. Alternate hydration measurements may be based on thin film ellipsometry using techniques developed for integrated circuit production to measure a thickness of the fluid film covering the corneal tissue surface.
In a first aspect the invention provides a method for measuring a surface topography of a surface of a tissue. The method comprises exposing the tissue to an excitation light energy so that the tissue produces a fluorescent light energy. The fluorescent light energy is measured from the fluorescent tissue, and the surface topography of the surface is determined using the measured fluorescent light energy.
Often times, the fluorescent tissue will be imaged onto a detector which is responsive to the fluorescent light energy. Preferably, the excitation light energy will be selected so that an amount in a range from about 50 to 100% of the excitation light energy is absorbed within a tissue depth equal to a resolution of the surface topography. The excitation light energy may be projected onto the tissue in a controlled irradiance pattern. The surface topography can be calculated from measured intensities of the fluorescent light energy.
A variety of excitation light energy wavelengths might be used, depending on the desired application. Generally, ultraviolet wavelengths in a range from about 150 to 400 nm, and more preferably from about 190 to about 220 nm are preferred for measuring exposed tissue surfaces. Similarly, while many wavelengths of fluorescent light energy can be measured, the measured fluorescent light energy from the tissue will generally be from about 250 to about 500 nm, the measured fluorescent light energy preferably being in a range from about 300 to 450 nm. Suitable excitation light energy sources include visible, ultraviolet, and infrared lasers, deuterium lamps, arc lamps, and the like. Typically, the excitation energy will have a different wavelength than the measured fluorescent light energy, allowing the excitation energy to be easily blocked from reaching the detector.
In another aspect, the invention provides a method for measuring a surface topography of an exposed surface of a corneal tissue. The method comprises making an excitation light energy with a wavelength in a range of about 190 to 220 nm. The tissue is exposed to the excitation light energy to induce a fluorescent light energy from the tissue. The fluorescent light energy has a wavelength in a range of about 300 to 450 nm. The excitation light energy is projected onto the tissue in a controlled irradiance pattern. From about 50 to 100% of the excitation light energy is absorbed by the tissue within a 3 xcexcm tissue depth from the exposed surface. The fluorescent light energy is imaged onto a detector responsive to the fluorescent light energy. An intensity of the fluorescent light energy is measured with the detector, and the surface topography is calculated from the measured intensity of the fluorescent light energy.
In another aspect, the invention provides a method for laser sculpting a region of a surface of a tissue. The method comprises directing an ablative light energy toward the surface, and inducing a fluorescent light energy from the tissue with the ablative light energy. An intensity of the fluorescent light energy is measured, and the shape of the exposed surface is determined using the measured intensity. The tissue is ablated with a pulsed beam of the ablative light energy.
In yet another aspect, the invention provides a system for measuring a surface topography of an exposed surface of a corneal tissue. The system comprises a light source generating an excitation light energy to induce a fluorescent light energy from the tissue. The excitation light energy has a wavelength in a range of about 190 to 220 nm, wherein about 50 to 100% of the excitation light energy is absorbed within a 3 xcexcm tissue depth so as to provide no more than 3 xcexcm resolution of the surface topography. A projection system projects the excitation light energy onto the tissue in a controlled irradiance pattern. An imaging system images the fluorescent light energy emitted by the tissue, and a spatially resolved detector measures an intensity of the fluorescent light energy emitted by the tissue in wavelength range of about 300 to 450 nm. A processor calculates the surface topography from the intensity of the fluorescent light measured by the detector.
In another system aspect, the invention provides a laser system for sculpting a region on an exposed tissue surface to a desired surface topography. The tissue has a threshold of ablation, and the system comprises a laser making a pulsed beam of an excitation light energy having an ablative wavelength that induces fluorescent light energy from the tissue. An optical delivery system delivers the light energy to the eye in a controlled manner to sculpt the surface. An imaging system images the fluorescent light energy, and a detector measures an intensity of the imaged fluorescent light energy to determine the shape of the exposed tissue.
In addition to topography measurements and topography-based laser ablation systems and methods, the invention also provides hydration measurement devices, systems, and methods for both measuring and selectively ablating tissues which are sensitive to their water content.
In a first hydration aspect, the invention provides a system for measuring hydration of a tissue. The system comprises a light source directing an excitation light toward the tissue so that the tissue generates fluorescent light. A fluorescent light sensor is in an optical path of the fluorescent light from the tissue. The sensor generates a signal indicating the fluorescent light. A processor is coupled to the sensor, the processor generating a hydration signal indicating the hydration of the tissue from the fluorescent light signal.
Many times, an ablation energy delivery system will be coupled to the processor. The delivery system will direct an ablative energy toward the tissue, and the processor will vary the ablative energy in response to the hydration signal. The tissue will typically comprise a corneal tissue of an eye, and the delivery system may comprise an optical delivery system transmitting photoablative laser energy toward the corneal tissue so as to selectively alter an optical characteristic of the eye. The processor may vary a quantity of change in the optical characteristic of the eye in response to the hydration signal. For example, the processor may vary a diopter value of the resculpting procedure in response to overall tissue hydration. Alternatively, the processor may vary the shape of the ablation by altering the ablative energy pattern so as to compensate for local differences in hydration across the target region of the corneal tissue. In some embodiments, an output device coupled to the processor may simply show a display in response to the hydration signal.
Generally, an intensity of the fluorescent spectrum of the tissue will vary with the hydration, so that the signal indicates an intensity of the fluorescent light at a first frequency. The processor will often normalize the signal using an intensity of the fluorescent light at a second frequency. The second frequency may be disposed adjacent a crossover point of a plurality of fluorescence spectrums of the tissue at different hydrations, so that the intensity of the fluorescent light at the second frequency is less sensitive to hydration than at the first frequency. Hence, the processor may calculate the hydration as a function of the relative intensity of the first frequency relative to the second frequency.
The sensor will often comprise a spectrometer, and imaging optics will often direct the fluorescent light along the optical path from the tissue to the spectrometer. The imaging optics may form an image of a target area of the tissue adjacent the spectrometer sensing surface.
In another aspect, the invention provides a system for use in an apparatus for resculpting a corneal tissue of an eye. The apparatus directs a pattern of light energy from a laser under the direction of a processor to effect a desired change in an optical characteristic of the eye. The system comprises a sensor coupled to the processor. The sensor generates a signal indicating hydration of the corneal tissue. An adjustment module of the processor varies the pattern in response to the hydration signal from the sensor.
In another aspect, the invention provides a method for measuring hydration of a tissue. The method comprises directing an excitation light energy toward the tissue so that the tissue generates fluorescent light. The fluorescent light is sensed, and the hydration of the tissue is calculated using the sensed fluorescent light.
In yet another aspect, the invention provides a compensation method for use in a procedure for resculpting a corneal tissue of an eye. The resculpting procedure will selectively direct a pattern of laser energy toward the eye to effect a predetermined change in an optical characteristic of the eye. The compensation method comprises sensing a hydration of the tissue. The pattern of laser energy is adjusted in response to the sensed hydration.
Typically, the hydration is sensed by directing an excitation light toward the tissue so that the tissue generates fluorescent light. An intensity of the fluorescent light is measured at a first frequency relative to a second frequency. The hydration of the tissue is calculated using the measured relative intensity. The ablation rate may be estimated for the calculated hydration, and the pattern adjusting step varied in response to this estimated ablation rate. Conveniently, the excitation light may be generated by the same source providing the ablative laser energy. Alternatively, the hydration may be sensed by measuring a thickness of a fluid film over the surface of the eye using ellipsometry.
In another method aspect, the invention provides a method for sculpting of a corneal tissue of an eye to effect a desired change in an optical property. The method comprises sensing hydration of the corneal tissue and determining a desired change in shape of the eye in response to the hydration, and in response to the desired change in optical property. A pattern of laser energy is planned for directing toward the corneal tissue, so at to effect the determined change in shape.
The desired change in optical quality will often be determined while the eye has a first hydration, optionally a normal hydration for the ambient conditions. The change in optical quality may be determined using any of a variety of standard vision diagnostic systems. Wavefront sensor systems now being developed may also be beneficial for determining a desired change in an optical property, and still further alternative topography and/or tomography systems may also be used. Regardless, rather than simply determining the desired change in shape of the eye from such measurements alone, the desired sculpting or ablation shape can also be based in part on the hydration of the eye.
Corneal tissue may increase in thickness by up to 50% due to changes in hydration by the time an ablation begins. Such swelling of the eye before and/or during an ablation procedure can be problematic, as the effective sculpting of the eye after hydration returns to normal can be significantly different than the intended result. More specifically, therapeutic compounds applied to the eye, incising of the eye to expose stromal tissue for a LASIK ablation procedure, and/or other standard techniques for preparation of and performing corneal sculpting may cause corneal tissue to swell like a sponge, significantly increasing both the hydration and thickness of corneal tissues. To effect the desired change in optical properties, a total depth of corneal tissue removal from the eye should be increased to compensate for such swelling of the corneal tissues.
In many embodiments, the corneal tissues may increase in thickness in a range from about 10% to about 50% with the increase in hydration. A first tissue removal depth which would effect the desired change in optical property of the eye when the eye has a first hydration (for example, at a normal hydration) may be increased by between about 10% and 50% when the eye has an enhanced second hydration (for example, during corneal ablation procedures). In many embodiments, the increase in tissue removal depth will compensate for swelling of the tissue, the increase depth percentage often being very roughly equal to the percentage of the swelling of the corneal tissue.